1. Field of the Invention
This invention relates to magnetostrictive sensors used to monitor stress, torque, relative torque changes, misfire, knock, pre-ignition, engine roughness, and the like, and more particularly, to an improved method and apparatus for such monitoring which has the added advantage of permitting shaft rotational position measurement in addition to magnetostrictive stress sensing.
2. Description of Prior Art
Magnetostrictive sensors take advantage of the magnetostrictive property of ferromagnetic materials whereby tension stress typically increases (and compressive stress decreases) a given magnetic induction field (i.e., the "B" field) carried by the material. Typically, a source for the magnetic induction field comprises a coil of wire of arbitrary number of turns wrapped around a ferromagnetic core and supplied with an appropriate level of electric current. The flux produced by the coil/core (the "primary" core/coil) passes through the material being stressed and hence changes with variations in stress levels within the material. Another one or more coil/core configurations (the "secondary" or "pickup" coil/core[s]) may then be used to monitor the magnetic flux changes using Faraday's law. Flux which varies with stress levels in the material passes through the secondary coil resulting (via Faraday's law) in voltage changes across the secondary coil. These voltage variations may then be correlated with stress levels in the material.
Many variations on this basic theme exist. For example, instead of a pickup coil, a Hall effect sensing element may be used. Instead of a primary coil/core, a permanent magnet may be used. In other configurations, the pickup and primary coil may be one and the same. Also, the magnetostrictive sensor may be used next to a shaft, torque disk, or other torque carrying member primarily to provide an indication of the level of torque in said member.
Several different designs of magnetostrictive sensor exist. These include, but are not limited to, the four branch design of FIG. 1, the cross design of FIG. 2, the two branch design of FIG. 3, the single branch design, and the solenoidal design. In SAE Technical Paper 900264, Fleming describes and compares these various designs and provides references to a considerable body of research work which has been done on magnetostrictive sensors.
FIG. 4 illustrates a device shown by Sugiyama in U.S. Pat. No. 4,697,460 wherein a magnetostrictive sensor is mounted adjacent the flywheel of an internal combustion engine instead of adjacent the shaft.
In their own prior work, shown in part in SAE paper #920236 and in PCT patent application PCT/US91/09280, the present inventors have tested magnetostrictive sensors extensively on automotive engines, and have devised a sensor system which monitors torque variations caused by misfire and other causes extremely well.
In addition to the value provided by a system which can monitor torque variations, automotive engine control units need a position sensor to indicate the instantaneous position of the engine crankshaft. This aids in setting proper timing and in other engine control and diagnostic applications.
Position sensors currently used are usually variable reluctance or Hall effect devices. These sensors are normally located adjacent an iron or steel ring gear which is tightly fitted around some portion of the driveline. The ring gear typically has teeth cut in it such that as the drive line turns, first one tooth, then a space between teeth, then another tooth, then another space, etc. passes sequentially under the sensor head. The sensor head emits magnetic flux such that the flux varies with the magnetic reluctance of the flux path, and the reluctance in turn varies depending on whether a tooth or a space is passing next to the sensor head. The change in flux is monitored as a change in voltage out of either a pickup core (variable reluctance type position sensor) or a Hall effect device.
At present, however, there is no known sensing system which includes the benefits of torque variation sensing employing magnetostriction along with position sensing wherein both torque and position may be sensed using the same sensor head. Hence the cost for performing these two monitoring functions is effectively twice the cost of monitoring either one.
There is, therefore, at present no known means for monitoring both magnetostrictive effects and shaft position using a single sensor head and saving almost half of the expense presently involved in performing both of these diagnostics.
In all of the work which has been done to date on magnetostrictive sensors, no suggestion or teaching exists which utilizes a magnetostrictive sensor for the additional function of monitoring position.